This invention relates to a composition comprising an aromatic amine compound and a small quantity of a stabilizing compound. The present invention also relates to a method for preventing the discoloration of aromatic amines, and to a process for the production of light colored aromatic amine based polyether polyols.
One of the problems or disadvantages associated with amine group containing compounds is the tendency of these compounds to discolor. Some amine group containing compounds such as, for example, ortho-toluenediamine (o-TDA), darken almost immediately upon exposure to air, while others such as, for example, aniline, are more stable and darken more slowly over time. The aliphatic amine group containing compounds also discolor over time at room temperature, but generally speaking, these discolor at a much slower rate than the aromatic amine group containing compounds.
Amine group containing compounds are known to be suitable initiators for preparing polyether polyols. Various amine initiated polyether polyols and the process for their production are described in, for example, U.S. Pat. Nos. 3,264,268, 3,314,995, 3,446,848, 3,462,492, 3,499,009, 4,209,609, 4,391,728, 4,421,871 and 4,562,290, and in British Patents 1,073,664, 1,311,095 and 1,398,185.
The use of discolored amine group containing compounds as initiators in the production of polyether polyols results in the polyether polyols also being discolored. Although it may be possible to reduce the dark color of the resultant polyether polyols, it is preferred that discoloration of these polyether polyols be prevented. Accordingly, a means of preventing discoloration of amine group containing compounds and/or reducing the color of polyether polyols started from amine group containing compounds is commercially desirable.
One way of avoiding/preventing discoloration of these amine group containing compounds and polyether polyols prepared from these compounds, is to immediately form polyether polyols from the amine group containing compounds after purification (usually distillation), before they come into contact with air. This, however, requires that the amine group containing compounds and the resultant polyether polyols be produced in the same plant or nearby plants, with little to no time lapse between the point at which the amine containing compounds are distilled and when these are used as initiators to form polyether polyols. Otherwise, stringent engineering measures are required to ensure that the amine compounds are oxygen-free at all points in the process between the time they are purified and used. Currently, it is necessary to keep the entire process after amine purification totally under positive nitrogen or another inert gas pressure to prevent and/or minimize this discoloration.
A process for the preparation of 2-mercaptotoluimidazole and metal salts thereof is described in U.S. Pat. No. 4,172,833. This process comprises reacting a stabilized raw material system comprising o-toluenediamine, a stabilizer, and carbon disulfide in the presence of a solvent, collecting the formed 2-mercaptotoluimidazole by filtration and recovering the solvent for recycling. Suitable stabilizers include hydrazine, hydrazine hydrate and hydrazine salts of strong acids. The addition of these stabilizers to o-toluenediamine prevents color degradation.
U.S. Pat. No. 4,877,879 relates to a process for stabilizing polyether compositions which are prepared from one or more alkylene oxides using an amine initiator. This process comprises contacting the crude polyether composition with a reducing agent in a sufficient quantity to effect the stabilization during or after neutralization. Formic acid and lithium borohydride are two reducing agents disclosed as being suitable. This process requires the addition of an excess amount of formic acid (or another reducing agent) at the end of the polyether reaction to neutralize the alkaline polymerizate. Although aromatic and aliphatic amines are disclosed as being suitable, it is further disclosed that this process is particularly useful when one or more tertiary amines are present in the initiator compound. N-aminoethylpiperazine, an aliphatic amine, is the preferred amine, and is used in all the working examples.
In accordance with the present invention, it was found that the addition of the presently claimed stabilizing compounds to an aromatic amine compound surprisingly formed compositions which are stable against discoloration. This is true even after storing the treated aromatic amine compositions in a 100.degree. C. oven for 4 weeks. Polyether polyols can then be produced from these treated aromatic amine compounds, without the derogatory color effects one would normally expect from an untreated aromatic amine compound.